The clocking circuits commonly used in electronic systems have traditionally derived their time bases directly from crystal oscillators, in order to exploit the high accuracy that is possible with a crystal oscillator. However, with continuing progress in the development of integrated circuits, the use of electromechanical components in electronic systems using integrated circuits has diminished greatly. The crystals used in oscillators are among the last of the electromechanical components used in electronic systems. Therefore, the crystal oscillator has become one of the least reliable components of most electronic systems.
At least three developments have occurred in the field of electronic circuits and systems which have made crystal-controlled clocking systems troublesome. Clocking rates, even in relatively inexpensive systems, have risen dramatically. Sending these high-frequency clock pulses coursing in conductors throughout an electronic system invites radio-like radiation and coupling of the clock pulses to functional signal conductors. Long conductors carrying very high frequency clock or timing pulses introduces a great potential for cross-talk and resultant disruption of the signal streams. Besides internal system problems from such radiation, the generation of and escape of such radiation might also exceed limits set by the U.S. FCC as well as the authorities of other countries.
The progress in microminiaturization of electronic components and circuits has further enhanced the potential for cross-talk of clock pulses into functional signal conductors by making the associated electrical components smaller. These smaller electronic components are thus more sensitive to small charges injected by cross-talk into their associated conductors.
As the integrated circuit components have gotten smaller, it would seem that their associated conductors would have gotten shorter and less likely to receive substantial amounts of cross-talk-linked electrical charge as a result of radiations from the high-frequency clock conductors. However, the sizes of systems have also grown. Therefore, the conductor lengths have not shortened as much as the component sizes have shrunk and the clock frequencies have increased.
One common approach to shortening the conductor paths that carry high-frequency clock pulses is to place a crystal oscillator next to each very large scale integrated circuit (VLSI). This can get expensive and uses an inordinate amount of circuit board area. The use of a great many individual crystal oscillators also multiplies the number of the less-reliable components that could fail and cause a system failure. That only compounds the reliability problem with the use of crystal oscillators. It also makes it difficult to control the phasing between the several VLSI chips in a system.
With so much integrated circuitry on a single printed circuit board, power budgeting can become a problem. If too many things happen simultaneously, current surges can be experienced by the various DC voltage sources that supply the printed circuit board.
There have been attempts to attack the crystal clock reliability problem by providing two or more crystal clocks. Then, various ways are found to choose the better of two--for example, by noting if one is going as much as half as fast as the other. Another approach is to rectify the output of the clock and look at the average voltage level of the rectified and filtered output. A low filtered output voltage means that the associated oscillator has either stopped or has slowed appreciably. Alternatively, three or more oscillators can be provided and their outputs compared. Then the two out of three that agree are selected, and one of those two is then used.
Most of the schemes that use multiple crystal oscillators and then switch between them pay little attention to the possible system consequences that might result from abrupt phase changes in the system clock.